1. Field of the Invention
This invention relates in general to orifice plate equipment for measuring fluid flow in pipelines, and in particular to a seal for sealing an orifice plate against the seat in an orifice fitting body.
2. Description of the Prior Art
One method of measuring fluid flow in pipelines is by the use of an orifice fitting. The orifice fitting is an assembly that will be mounted for receiving fluid flow. It has a body with a passage for the flow of the fluid. A gear mechanism will lower an orifice plate down into the fluid flow passages. The orifice plate has a hole through it. Fluid flow through the hole causes a pressure drop. The difference in pressure upstream and downstream of the plate is used to calculate the fluid flow volume.
A plate carrier carries the orifice plate. The orifice plate has a seal which seals on the downstream side of the orifice plate. The seal seals against an annular seat which surrounds the flow passages. It is important to have good sealing so as to obtain an accurate measurement of the pressure drop.
In the prior art, the seal is an oversized elastomeric ring surrounding the orifice plate. It compresses as the plate carrier forces it into a slot between the seats. The slot will have a lesser width than the width of the elastomeric seal ring surrounding the orifice plate.
A disadvantage of this technique is that the seal ring may be easily cut or damaged when forced into the seat slot if the protrusion of the seal is too great. Also, variations in face-to-face dimensions between the seats as well as corrosion and erosion of the seat areas often cause leaks past the seal ring. In larger orifice fittings, unsupported segments of the seat area will sometimes expand beyond the yield strength of material during pressure testing and will expand during normal operations. This causes a face-to-face dimension in this portion of the seat slot that is in excess of the allowable limits. This may result in leakage.
Because the leakage around the plate occurs only from an upstream to a downstream side of a plate, and not to the exterior of the fitting, it is difficult to detect. The seat slot dimension is affected by internal pressure and temperature, both inside and outside. The seat slot dimension in orifice fittings is critical in that leakage across the orifice plate-to-body seal causes significant error in measurement of fluid volume flowing through the orifice fitting.